The invention relates to a method of applying a corrosion-proof and wear-resistant coating to a workpiece, and in particular to the piston rod of a hydraulic ram of the type used in underground mining operations or in tunnel driving.
It is well known that galvanic or currentlessly-precipitated coatings of a metal are usually effective only against specific corrosives. It is also known to coat the piston rods of hydraulic pit props (or other hydraulic rams used in underground mining) with chromium using the hard plating process. Such a chromium coating layer is relatively brittle, so that the danger exists that, in use, it may break. Consequently, it is usual to apply galvanically a substantially crack-free chromium layer to the piston rod prior to applying the chromium plating using the hard plating process. The galvanically-applied layer forms an underlayer for the coating, which takes the form of a microcracked hard-chromium layer. Hydraulic rams provided with such a combined protective coating have proved their value in mining operations. The hard-chromium plating is distinguished by high wear resistance, and also by stability to corrosion in a reducing corrosive medium (such as a sulphur dioxide industrial atmosphere). On the other hand, the corrosion stability of the hard-chromium plating in relation to an oxidising atmosphere (and especially in relation to a chloride-containing atmosphere) is inadequate. Here, under certain conditions (such as are to be encountered in underground mining), serious damage can occur to the hard-chromium-plated parts, as early as within a few days or weeks. In particular heavy pitting can occur, so that damaged props and rams have to be taken out of use and replaced.
There is a tendency for protection against a predominantly reducing atmosphere to lose importance in underground mining. For some time, pit atmospheres have been modified towards a chloride-containing oxidising atmosphere. This is attributable to various influencing factors, but mainly to the intensification of the measures for combatting dust in underground workings.
Until now, it has not been possible to develop a protective coating for props (and other hydraulic rams), which coating has high wear-resistance and is corrosion-proof in relation to both a reducing atmosphere and a chloride-containing oxidising atmosphere.
It is known to apply a single-layer or multi-layer protective nickel coating to the piston rods of pit props (or other hydraulic rams) using the known currentless nickel-coating process. This process is based upon a reduction of the nickel ions present in the nickel bath using a suitable reducing agent. The known hypophosphite process, in which a hypophosphite (such as sodium hypophosphite) is used as a reducing agent, has proved its value especially well here. With this process, it is possible to form protective coatings, with a variable phosphorus content, which are distinguished by high density and extensive pore-free quality, and also by relatively high hardness, tensile strength and breaking elongation (ductility). These currentlessly-applied protective nickel coatings are also corrosion-proof in relation to a chloride-containing oxidising atmosphere. Unfortunately, their corrosion resistance in relation to a reducing sulphur dioxide atmosphere is inadequate.
Attempts to apply a wear-resistant, hard-chromium coating to a currentlessly-applied nickel underlayer have hitherto been unsuccessful, since the adhesion of the hard-chromium coating upon the nickel layer is inadequate. Consequently, in use, the hard chromium coating tends to flake away from the nickel layer.
The aim of the invention is to produce coatings for the piston rods of pit props (or other hydraulic rams) which have high wear resistance, and display a high corrosion stability both in relation to a reducing atmosphere and in relation to an oxidising chloride-containing atmosphere.